The present invention relates to a developer which is used to develop an electrostatic latent image in an electrophotographying method or the like and, more particularly, to a developer in which two components of the toner and carrier are mixed.
Hitherto, as an electrophotographying method, a method disclosed in U.S. Pat. No. 2,297,691 or the like has been known. According to the above electrophotographying method, in general, a photoconductive insulating material (photocon drum or the like) is used, uniform electrostatic charges are given onto the photoconductive insulating material by a corona discharge or the like and a light image is irradiated onto the photoconductive insulating material by various kinds of means, thereby forming an electrostatic latent image. Subsequently, by developing the latent image by using fine powder called a toner, its visual image is derived. The toner image is copy transferred onto a paper or the like as necessary. The toner image is fused by using a pressurizing method, a heating method, a solvent vapor, a light, or the like and is fixed onto the paper or the like, thereby obtaining a printed matter.
As toner to develop the electrostatic latent images, hitherto, a colorant or the like such as carbon black or the like is dispersed into a binder resin made of a natural or synthetic high polymer material and the resultant resin is finely ground so as to have diameters of about 1 to 30 .mu.m and the resultant particles are used as such a toner. The toner is ordinarily mixed to the carrier such as iron powder, glass beads, or the like and is used to develop the electrostatic latent image. In case of using the iron powder or other ferromagnetic particles as carrier, the developer consisting of the toner and carrier is mixed and stirred in a developing apparatus, so that it is frictionally charged. The charged carrier is further held by a magnet roll in the developing apparatus and forms a magnetic brush. By rotating the magnet roll, the magnetic brush is carried to the latent image portion on the photoconductive insulating material. Only the charged toner is deposited to the latent image by an electrical attracting force, thereby performing the development.
In such a conventional developer, however, when it is used in an environment in a range from a low temperature of 10.degree. C. and a low humidity of 20% RH (RH: an abbreviation of relative humidity) to a high temperature of 30.degree. C. and a high humidity of 80% RH, a friction charging property of the developer changes to a state of an ordinary temperature and an ordinary humidity, so that there is a case where such that print blur, print blank, or background fog occurs. To solve such a problem, devises have conventionally been made with respect to the developers. For instance, in an environment of a low temperature and a low humidity, a charging amount of the developer increases, so that there is a case where a print density decreases, a print blur occurs, or a print blank due to the deposition of the carrier onto the photo sensitive drum occurs. Since the charging amount of the developer decreases in the high-temperature and high-humidity environment, a fog such that an unnecessary image is generated in the background portion of the paper occurs. There is also a case where a resistance of carrier decreases because of the deposition of the moisture of the developer, the inverse charging toner is generated due to the injection of charges into the toner from a developing roller and appears as a fog in the background portion of the photo sensitive drum although no background fog occurs on the paper, so that the toner is consumed in vain.
On the other hand, hitherto, a fluctuation of charging amount in a low-temperature and low-humidity environment or a high-temperature and high-humidity environment has been solved by using the carrier such that the magnetic powder is coated by a resin of the fluorine system or silicon system of a high hydrophobic property. To prevent the generation of the inverse charging toner due to a decrease in electric resistance in a high-temperature and high-humidity environment, the carrier of a high resistance of 10.sup.10 .OMEGA.cm or higher is used, thereby preventing the injection of the charges from the developing roller.
In recent years, however, the realization of a low end of the printer and its personalization have rapidly been being progressed. The use environment is further extended to a wide range. As a printer, stable printing characteristics in a range from an environment of a superlow temperature of 0.degree. C. and a low humidity of 0% RH to an environment of a superhigh temperature of 40.degree. C. and a high humidity of 80% RH are demanded. In the conventional developer, on the other hand, a charging amount of the developer certainly rises in the low-temperature and a low-humidity environment, so that a print blur or carrier deposition occurs. On the other hand, there is also a case where in the high-temperature and high humidity environment, a background fog occurs on the paper due to a decrease in charging amount. Namely, to assure a print density in the superlow-temperature and low-humidity environment of 0.degree. C. and 0% RH, it is inevitable to avoid a resistance value of the carrier.
It has been found out that the print blur or carrier deposition in the superlow-temperature and low-humidity environment of 0.degree. C. and 0% RH are solved by setting the electric resistance of the carrier to 1.times.10.sup.8 .OMEGA.cm or less. However, when the resistance of carrier is reduced, on the contrary, a resistance value decreases due to a blur of the paper or the deposition of the moisture of the carrier because of the reduction of the charging amount of the developer in the environment of a superhigh temperature of 40.degree. C. and a high humidity of 80% RH, so that a background fog of the photo sensitive drum occurs and deteriorates.
Such a decrease in charging amount of the developer and a decrease in electric resistance of the carrier due to the moisture deposition are not solved even by using the conventional carrier obtained by coating the resin of the fluorine system or silicon system of a high hydrophobic property to the magnetic powder. Namely, there is a limitation when those methods are used. In case of using the coating carrier of the silicon resin system of a high hydrophobic property, as compared with the carrier which is not coated, an image is preferably seen even for a fog until a high-temperature and a high humidity environment of about 30.degree. C. and 80% RH. It has been found out, however, that there is hardly an effect in the environment of a high temperature of 40.degree. C. and a high humidity of 80% RH and a fog occurs on the whole region of the background of each of the paper and the photo sensitive drum.
It is an object of the present invention to provide a developer which is used in the electrophotographying method and which can solve the occurrence of a fog even in a use environment in a range from a superlow temperature and a low humidity and a superhigh temperature and a high humidity and also to provide a method of manufacturing such a developer. More particularly, another object of the invention is to provide a carrier as an intermediate material and its manufacturing method.